Harvest index, ratio of grain to total aboveground biomass, has remained nearly constant around 50% in maize over the past 100 years. (Sinclair, (1998) “Historical changes in harvest index and crop nitrogen accumulation”; Crop Science 38:638-643); (Tollenaar and Wu. (1999) “Yield improvement in temperate maize is attributable to greater stress tolerance”; Crop Science 39:1597-1604). Thus, the quadrupling of grain yield over the last 50-60 years has resulted from an increase in total biomass production per unit land area, which has been accomplished by increased planting density (Duvick and Cassman, (1999) “Post-green revolution trends in yield potential of temperate maize in the north-central United States”; Crop Science 39:1622-1630). Selection for higher grain yield under increasing planting densities has led to a significant architectural change in plant structure, that of relatively erect and narrow leaves to minimize shading. An undesirable consequence of denser planting has been the increased frequency of stalk lodging. The relationship between planting density and biomass production deviates significantly from linearity as the optimal density is approached for maximal biomass yield per unit land area. This is reflected in a proportionately greater reduction in the individual plant biomass, which manifests in the form of weaker stalks and hence increased lodging.
Cellulose in a unit length of the maize stalk was found to be the best indicator of mechanical strength (Appenzeller, et al., (2004) “Cellulose synthesis in maize: isolation and expression analysis of the cellulose synthase (CesA) gene family”; Cellulose 11:287-299; Ching, et al., (2006) “Brittle stalk 2 encodes a putative glycosylphosphatidylinositol-anchored protein that affects mechanical strength of maize tissues by altering the composition and structure of secondary cell walls”; Planta 224:1174-1184). Cellulose constitutes approximately 50% of the stalk dry matter at maturity (Dhugga, unpublished). Whereas the concentration of cellulose in the maize stalk wall can vary considerably in the germplasm that for lignin is essentially invariable (Dhugga, unpublished). This implies that the concentration of cellulose varies at the expense of other cell wall components such as hemicellulose and soluble components. Increasing cellulose concentration in the dry matter should allow improving harvest index without adversely affecting stalk mechanical strength. Thus the most preferred means of improving stalk strength are through increased cellulose concentration and secondary wall content.
This invention pertains to a set of maize genes involved in cell wall formation, in particular secondary cell wall formation (SCW). Cellulose may constitute up to 60% of the secondary cell wall of plants such as maize. The genes that are subject of this invention are revealed to be associated with secondary cell wall formation based on the strong correlation of their expression patterns with those of ZmCesA10, ZmCesA11, ZmCesA12, ZmCesA13, and Bk2 genes that had previously been shown to be involved in secondary cell wall formation (Appenzeller, et al., (2004) “Cellulose synthesis in maize: isolation and expression analysis of the cellulose synthase (CesA) gene family”; Cellulose 11:287-299; Ching, et al., (2006) “Brittle stalk 2 encodes a putative glycosylphosphatidylinositol-anchored protein that affects mechanical strength of maize tissues by altering the composition and structure of secondary cell walls”; Planta 224:1174-1184; Dhugga, unpublished). Many of the genes in this invention are not known to be associated with cell wall formation outside of these proprietary analyses. These genes could be used to enhance crop plant performance and value in several areas including: 1) plant standability, harvest index, and yield potential; 2) plant dry matter as a feedstock for ethanol or for other renewable bioproducts; and 3) silage.
In addition to its role as the primary determinant of tissue strength, a trait that is of significant interest in agriculture, cellulose constitutes the most abundant renewable energy resource on Earth. Approximately 275 million metric tons of stover is produced just from maize in the USA every year. About two-thirds of stover could potentially be utilized for ethanol, butanol and other fuels or bioproducts from some corn-growing regions (Graham, et al., (2007) “Current and potential U.S. corn stover supplies”; Agronomy Journal 99:1-11). The worldwide production of lignocellulosic wastes from cereal stover and straw is estimated to be ˜3 billion tons per year (Kuhad and Singh (1993) “Lignocellulose biotechnology: Current and future prospects”; Critic. Rev. Biotechnol. 13:151-172). Secondary wall accounts for a great majority of the vegetative biomass in the terrestrial vegetation and is thus a suitable target for manipulation to improve the amount and quality of biomass for energy production. Alteration of secondary wall for improved silage quality may be accomplished by altering lignin concentration. Both the type and amount of lignin have long been known to affect silage digestibility. Lignin is also an impediment in the digestion of cell wall polysaccharides for ethanol production. Several of the genes in our list expand the number of candidates we can use to alter the composition of cell wall for improved silage quality.
This invention provides solutions to agronomic problems in at least three areas: 1) plant standability, harvest index, and yield potential; 2) plant dry matter as a feedstock for ethanol or for other renewable bioproducts; and 3) silage digestibility.